Transcription of eukaryotic genes by RNAPII requires the assembly of multiple protein factors into a transcription initiation complex. In addition, the regulation of this process with respect to the selective activation of specific genes and the rate at which these genes are transcribed is often dependent upon interactions of protein regulatory factors with this complex. Herpes simplex virus is used as a model system for the analysis of the genetic sequences and protein factors required for the regulated transcription of mammalian genes by RNAPII. In the course of a viral infection, three classes of genes (Immediate early, Early, and Late) are transcribed in a distinct temporally regulated manner. Of these, the Immediate early genes are regulated by the interactions of viral (TIF) and host cell proteins (Oct-1 and C1 factor). Several approaches have been used to identify and analyze the domains and interactions of these factors which are required to regulate the initiation of IE gene expression. The continuing study is involved in the development of an in vitro transcription system which reproduces the regulatory requirements of each viral gene class. This will allow for the analysis of the basic and specific protein factors which selectively activate a particular gene class as well as elucidation of the biochemical mechanisms involved in this process.